There are two primary types of natural vibrations (i.e., resonant oscillations) associated with the blade of a wind turbine. Flapwise vibrations occur in a plane perpendicular to leading and trailing edges of the blade. Edgewise vibrations occur in a plane through the leading and trailing edges. Both types of vibrations place significant loads on the blade that can intensify fatigue damage and lead to failure. Therefore, it is important to avoid exciting these vibrations.
This is particularly true when a blade enters an operational condition called stall. During stall, the airflow over the upper surface of the blade becomes increasingly turbulent. If turbulence or other factors excite the blade's natural vibrations, aerodynamic forces tend amplify these movements. This occurs because of a principle called negative aerodynamic damping.
There is a high risk of damage in the situation described above, especially in stalled-controlled turbines where stall is intentionally used to control power output. Specifically, the aerodynamic forces that excite natural vibrations during operation are a function of the blade's tip speed squared. These forces are significant during stall because that condition occurs at relatively high wind speeds.
Pitch-controlled turbines do not experience the situation described above as much as stalled-controlled turbines. This is because the blades of a pitch-controlled turbine can be pitched to change the aerodynamics when negative aerodynamic damping is detected. Nevertheless, the situation may still occur for a brief period of time. The blades of a pitch-controlled turbine may also experience the amplification of natural vibrations when “parked” during a storm with extremely high winds. In such a situation, however, the aerodynamic forces exciting the natural vibrations of the blade are a function of the wind speed squared.
There are two main principles that counteract negative aerodynamic damping: 1) another aerodynamic principle known as dynamic stall, and 2) structural damping. Although dynamic stall plays an important role in reducing flapwise vibrations, it is only slightly effective in reducing edgewise vibrations. Therefore, the primary factor in preventing edgewise vibrations is a blade's structural damping.
Several ways to increase the damping of a structural blade have been developed. For example, WO 95/21327 discloses a blade having an oscillation-reduction element oriented in the direction of unwanted oscillations. Although the patent application first describes the oscillation-reduction element using generic terms and depicts it using conventional symbols, most of the embodiments disclosed are tuned liquid dampers. These dampers are specifically designed (i.e., “tuned”) to have a natural frequency substantially corresponding to the dominating natural frequency of the blade. As such, their effectiveness at damping vibrations is frequency-dependent. They also typically require maintenance and can be difficult to access and install.
Passive dampers are also known. One example of a passive damper is disclosed in WO 99/43955. However, because passive dampers are typically difficult to design and implement, the number of adequate solutions developed has been limited. There remains plenty of room for improvement in this area.